1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a power transmitting system for automotive vehicles having a belt-and-pulley type continuously variable transmission.
2. Discussion of the Prior Art
In the art of an automotive vehicle, there is known a power transmitting system having a belt-and-pulley type continuously variable transmission, which includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys, and a pair of hydraulic actuators for changing effective diameters of the pulleys. The transmission is controlled by a hydraulic control apparatus which includes a directional control valve for supplying a working fluid from a pressure line selectively to one of the two hydraulic actuators while discharging the fluid from the other hydraulic actuator, for automatically controlling a speed ratio of the transmission. An example of this type of continuously variable transmission is disclosed in laid-open Publication No. 62-196445 of unexamined Japanese Patent Application.
In a motor vehicle equipped with such a continuously variable transmission whose speed ratio is automatically changed during running of the vehicle, it is desirable that the shifting of the transmission to change the speed ratio for reducing the vehicle running speed be effected rapidly in a comparatively long time, since the shifting is required to increase the driving power of the vehicle. On the other hand, the shifting of the transmission to change the speed ratio for increasing the vehicle speed is desirably effected slowly in a comparatively long time, since this shifting tends to suddenly accelerate the vehicle with a drive force due to inertia of the engine and the associated components, which deteriorates the driving comfort.
In the conventional hydraulic control apparatus, the directional control valve is connected to the driving and driven side actuators (provided on the driving or input and driven or output shafts of the transmission), through respective two fluid passages which have the same resistance to flows of the working fluid therethrough. In this arrangement, the shift-up speed and the shift-down speed of the transmission are the same. That is, the rate at which the speed ratio of the transmission is increased is the same as the rate at which speed ratio is reduced. Thus, the shift-up and shift-down speeds cannot be adequately controlled in the conventional arrangement. Although it is possible to control the rates of flows of the fluid to the driving and driven side actuators to different optimum values, this requires a complicated electric controller for controlling the relevant component or components of the hydraulic circuits.
Laid-open Publication Nos. 62-196448 and 62-196450 of unexamined Japanese Patent Applications disclose a hydraulic control apparatus which includes a shift control valve assembly for controlling the speed ratio of a belt-and-pulley type continuously variable transmission as described above, such that a first and a second line pressure is applied from a first and a second pressure line to one and the other of the driving and driven side hydraulic actuators, respectively. In this hydraulic control apparatus, there is provided a by-pass line which connects the driven side actuator and the second pressure line. The by-pass line is provided with a flow restrictor or a one-way valve, which serves to avoid a drop of the pressure in the driven side actuator due to leakage of the fluid therefrom, and thereby prevent the transmission belt from slipping on the pulleys due to insufficiency of the pressure in the driven side actuator.
The shift control valve assembly may include a two-position directional control valve and a flow control valve whose duty cycle is controlled so as to regulate the rate of flow of the fluid. In this case, the driven side actuator suffers from pressure pulsation which occurs in synchronization with the duty-cycling operation of the flow control valve. This pressure pulsation causes pulsation of the tension of the transmission belt which is determined by the pressure in the driven side actuator. Even if the pressure in the driven side actuator is regulated to an optimum value, the pulsation causes fluctuation of the belt tension, between two values corresponding to the upper and lower peaks of the pressure pulsation of the driven side actuator. This results in reducing the life expectancy of the belt. If the flow restrictor is provided in the by-pass line, as described in the above-identified laid-open Publication No. 62-196448, the flow restrictor should not allow a significant amount of leakage of the fluid from the driven side actuator while the flow control valve is not operated in the duty-cycling manner. This means that the flow restrictor is not sufficiently effective to release the upper peak of the pressure pulsation of the driven side actuator upon duty-cycling operation of the flow control valve. If the one-way valve is provided in the by-pass line, as described in the above-identified laid-open Publication No. 62 -196450, the one-way valve is only capable of releasing the upper peak of the pressure pulsation of the driven side actuator, but is not capable of applying the second line pressure to the driven side actuator, so as to compensate for the lower peak of the pulsation.
Laid-open Publication No. 60-95262 discloses a hydraulic control apparatus, wherein a first solenoid coil is used for operating the directional control valve selectively to a first and a second position thereof for increasing or reducing the speed ratio of the transmission, while a second solenoid coil is provided for operating the flow control valve selectively to a first and a second position thereof for changing the speed ratio of the transmission at one of two different rates. The first and second solenoid coils are energized or deenergized so as to control the directional and flow control valves so that the actual speed of the driving or input shaft or the actual speed ratio of the transmission coincides with a desired or target value.
In the hydraulic control apparatus as described above, the speed ratio of the transmission is controlled by applying a first line pressure to one of the driving and driven side actuators and applying a second line pressure lower than the first line pressure to the other of the two actuators, while the tension of the transmission belt is controlled by the second line pressure applied to the driven side actuator If the directional and flow control valves which are controlled by the first and second solenoid coils as described above are used in this case, the spools of these control valves tend to be large-sized and have relatively large masses, which deteriorate the operating response Further, where the flow control valve is operated in the duty-cycling manner so as to continuously change the flow rate of the fluid, the large mass of the spool of the flow control valve is undesirable for the durability of the valve.
Laid-open Publication No. 60-95262 discloses a hydraulic control apparatus, wherein a flow control valve for regulating the rate of change in the speed ratio of the transmission has an open position and a closed position for respectively selecting a first rapid shifting mode and a second slow shifting mode, so that the speed ratio is rapidly changed in the rapid shifting mode, and slowly changed in the slow shifting mode. Further, the duty cycle of the flow control valve is controlled so as to provide a third medium-speed shifting mode in which the speed ratio is changed at a continuously variable rate intermediate between the rates of change in the first and second shifting modes. The speed ratio of the transmission is controlled in a selected one of the first, second and third shifting modes depending upon the amount of the detected control error associated with the speed ratio, so that the control error is zeroed or so that the actual speed ratio coincides with a desired or target speed ratio.
In the above hydraulic control apparatus., the slow shifting mode is selected when the currently detected control error amount is relatively small, and the medium-speed shifting mode is selected when the control error amount is medium. When the control error amount is relatively large, the rapid shifting mode is selected. In the medium-speed shifting mode, the duty cycle at which the flow control valve is operated is determined based on the detected control error amount, according to a predetermined formula or equation.
The flow rate of the fluid regulated by the flow control valve does not effectively vary with a change in the duty cycle of the valve, when the duty cycle is near 0% and 100%. Namely, the duty cycle of the flow control valve has non-controllable ranges in which the flow rate of the fluid regulated by the flow control valve does not change with the duty cycle, as in a controllable range in which the flow rate effectively changes as the duty cycle changes. The non-controllable ranges exist due to inertia of the spool of the flow control valve, inductance of the solenoid coil for operating the flow control valve, viscosity and compressibility of the working fluid, and other factors This causes a problem in controlling the rate of change in the speed ratio of the transmission. For example, if the slow shifting mode is replaced by the medium-speed shifting mode, based on the detected or determined control error, the duty cycle determined based on the control error according to a predetermined formula immediately after the selection of the medium-speed shifting mode does not necessarily accurately correspond to the boundary between the controllable and non-controllable ranges of the duty cycle of the flow control valve. That is, the duty cycle may considerably differ from the value corresponding to the boundary, due to fluctuation in the gain used in the formula. In this case, the flow rate regulated by the flow control valve immediately after the medium-speed shifting mode is selected is considerably different from that at the end of the shifting in the slow shifting mode. This results in a sudden change in the speed of changing the speed ratio, which deteriorates the driving comfort of the vehicle.